Abstract The optical properties of intrinsic hydrogenated amorphous silicon, p-type hydrogenated amorphous silicon-carbon, and tin oxide films are measured as a function of wavelength. Computer simulations based upon these and other measurements illustrate the optical development of hydrogenated amorphous silicon solar cells and quantify losses associated with each layer. The basic structure considered is glass/tin oxide/p-i-n/metal where the tin oxide film may be either specular or granular to promote light trapping. Methods for reducing the reflection loss and increasing the reflectivity of the back metal contact are discussed. Upper bounds of about 12.5% efficiency are predicted for single function a-Si:H based solar cells. The difference in performance between small and large area solar cells as well as the different optimum tin oxide thicknessesor these cells is also discussed.